1. Field of the Invention
The present invention relates to a printed circuit board suitable for dip soldering of component leads in through holes of the printed circuit board using lead free solder, and a method and apparatus for dip soldering.
2. Description of the Related Art
Dip soldering that uses Sn—Pb eutectic solder is a common method of soldering component leads in through holes of printed circuit boards. A power supply terminal pattern or ground pattern on the surface of a printed circuit board consisting of copper foil 31 usually stretches over a wide area in a continuous manner as shown in FIG. 6A and FIG. 6B. A through hole 33 with a copper foil 34 on its inner surface is provided in a portion where a component lead 32 is connected, so that the lead will be inserted in the through hole and soldered to connect an electronic component thereto. The copper foil 31 on the surface includes an annular portion 35 around the through hole 33 with a predetermined width, a plurality of circular arc slits 37 with a predetermined width connected by a plurality of circumferentially spaced connection parts 36 around the annular portion. This is called a thermal land 38, and provided for quickly conducting the heat of molten solder that has flowed into the through hole 33 to the copper foil 31 on the surface to quickly cool down the solder so that it does not reach up to the upper surface of the through hole 33; it is intended to ensure formation of a favorable fillet between the periphery of the lead 32 and the copper foil 31 on the surface. Heat is conducted to the annular portion 35 around the through hole 33, which further assists the formation of the fillet.
In a known printed circuit board technique shown, for example, in Japanese Patent Publication No. 9-148691, planar patterns of copper foil are formed on the upper and lower surfaces of a portion where a heat generating element is mounted, separately from a circuit pattern for connecting the connection terminal of the heat generating element. Six or more via holes with copper-plated inner walls are drilled through the upper and lower surfaces of this portion in a dense arrangement for the purpose of heat conduction. Both ends of the via holes are respectively connected to the upper and lower copper foil planar patterns, so that heat from the heat generating element is efficiently dissipated from the upper and lower planar patterns.
Meanwhile, lead free solder has been increasingly used in soldering operations of printed circuit boards so as to avoid adverse effects of heavy metal such as lead on the environment. However, dip soldering operations using lead free solder with the printed circuit board design shown in FIG. 6A and FIG. 6B entail various problems, such as insufficient solder wicking up into the through holes and the consequent difficulty of forming fillets, air entrapment in solder, and short heat fatigue life of solder joints.
The technique disclosed in Japanese Patent Publication No. 9-148691 only shows a design which combines the upper and lower copper foil planar patterns with a large number of via holes with copper-plated inner walls for the purpose of dissipating heat from the heat generating element and does not teach direct solutions to such problems.